The fundamentals of the above mentioned problems are well understood, because the origin of these problems is very similar to the well-known “viewing angle” problem found in liquid crystal displays (LCDs) in general. Because of the very large impact of the viewing angle problem, found in liquid crystal displays (LCDs) of laptop computers and TV, worldwide interest in this problem has led to a number of solutions.
The origin of the “viewing angle” problem, as presented in the FIGS. 1 and 2 can be in general regarded as threefold: a    1. Effective birefringence of the homeotropically aligned LC molecules in the central part 3 of the LC layer 4 of the LCD in the closed state of the LCD light-switching element, when observed at an oblique angle,    2. Effective overall birefringence of thin transition surface layers 2a and 2b of the LC molecules 4 at both boundary surfaces 1ca, 1cb of the LCD, which are relatively little affected by the driving electric field, but rather controlled by the alignment surface interactions,    3. Effective birefringence asymmetry resulting from the asymmetry of the LC molecular alignment in the transition surface regions 2a and 2b—strongly depends on the twist angle used in the particular structure of the LCD.
Finally the LCDs typically employ crossed polarizers to detect the changes in the birefringent properties of the LC layer under the effect of the driving electric field. As has been well know for decades (L. Baxter: J. Opt. Soc. Am., 46, p. 435) the polarizers themselves exhibit noticeable angular dependence, as the effective angle between the crossed polarizers is dependent on the light incidence angle. The problem of changing the effective angle between the crossed (90°) polarizers with the viewing angle has been rather recently very efficiently solved by P. Bos at al (Jpn. J. Appl. Phys. Vol. 38 (1999), SID Dig. (1998) p. 315), by using two additional, adequately positioned birefringent layers—positive-birefringent a-plate and positive-birefringent c-plate retardation layer, placed between the crossed polarizers. The problem however is that from a technical point of view lamination of two rather special additional retardation layers into the LCD assembly is rather expensive and time consuming.
Aside from the above problem of the crossed polarizers, as described above, the general problem of the angular dependence of LCDs has been given a tremendous attention by the LC research, as well as by the LCD industrial producers' community and large number of different technical solutions has been developed using different twist nematic (TN) LCD concepts (standard TN (twist angle=90°), low TN (twist angle≈70°), super TN (twist angle—significantly higher than 90°—typically 240°, . . . ). In general each of the wide viewing angle technical solutions is based on the additional use of some kind of the birefringent compensation layer incorporated between the LCD cell and the polarizers. “Negative birefringence films” necessary to compensate for the “positive” birefringence of standard nematic LC, were sought after unsuccessfully for a number of years, with one of the first successful results shown by Uchida (SID 89 DIGEST, p 378-381) from Tohoku University in 1989 and Clerc (U.S. Pat. No. 4,001,028, U.S. Pat. No. 4,889,412, U.S. Pat. No. 5,298,199) from Stanley in 1991. results were based on the shear induced negative birefringence in a thermoplastic material cycled through the glass transition. Few years later Eblen from Rockwell showed that a multilayer thin film can have negative birefringence properties (U.S. Pat. No. 5,196,953) and very recently Pirs et al (WO 00/77561 A2) obtained similar negative birefringent properties by a mechanical stress induced in a thin layer of polymer by a fast, controlled polymerization process. Yet another process was demonstrated by Harris and Cheng in 1994 and later by Shin-Tson-Wu from Hughes (U.S. Pat. No. 5,344,916, U.S. Pat. No. 5,580,950, U.S. Pat. No. 5,480,964). They showed by spinning down a thin layer of a particular class of pre-imidized polyimides, that the long molecular segments would align preferentially in the plane of the film and therefore a higher value of the index of refraction results for light propagating along the cell normal than at an angle to it. Their material was one of the first films that were rather convenient to produce. Finally one has to mention a number of excellent technical solutions developed for high information content LCD panels (computer terminals, TV, . . . ) like compensating films for STN LCDs, as developed by NITO Denko (U.S. Pat. No. 5,245,456, SID Digest 92, p 739 and SID Digest 91, p 739) and polymer discotic LC compensating layers developed by Fuji Film (U.S. Pat. No. 5,559,618, U.S. Pat. No. 5,646,703, U.S. Pat. No. 5,525,265) as well as twisted nematic polymer retardation layer developed by Akzo Nobel (U.S. Pat. No. 5,382,648, U.S. Pat. No. 5,525,265).
A point to be emphasized here is, that due to the voltage limitations, that exist for picture elements in the multiplex addressed LCD screens of lap-top computers and TV, as well as the overall user requirements for these display panels, the evolution of films to improve the viewing angle in this case may not lead to the desired performance and cost target for a shutter device, that has to be used for example in the personal protection devices.
In any case the residual overall positive birefringence of the LC molecules in the optically closed state of the LCD light-switching elements represents the major origin of the inadequate off-normal axis performance and the major correction can be achieved through the introduction of the negative birefringent films. However, there are noticeably different overall requirements for these “negative birefringent compensatory films” in the case of “laptops” and “light shutters”. In the case of a shutter device for personal eye protection applications (e.g.: welding glasses, . . . ), the drive voltage can be two to four times that of a LCD for a lap-top computer, and it is much more critical, that very high light attenuations are reached and the off-normal axis light extinction properties of the device are excellent. The relatively high driving voltage typically used with LCD light-switching elements, like shutter devices for personal eye protection applications, result in very good homeotropic alignment of the great deal of the LC layer in the LCD light-switching element. Therefore complex, expensive compensation layers, as mentioned above to be developed by Fuji-film for high information content LCDs in computer terminals, are not necessary and simple, cheap negative c-plate retardation compensation layers seem to be completely adequate.
Besides the angular dependence, resulting from the homeotropically aligned central LC layer 3 in the optically closed state of the LCD light switching optical element, one has to consider also the angular dependence, resulting from the residual overall birefringence of the LC molecules in the transition surface LC layers 2a and 2b, as well as the optical asymmetry of these layers. The former results in the shift of the maximum attenuation direction off the normal light incidence axis—not acceptable for the light switching elements used in personal protection applications as well in stereovision. Finally the asymmetry of the optical birefringence of the LC molecular structure in the transition surface LC layers 2a and 2b result in poor angular dependence symmetry of the light attenuation around the axis normal to the LCD plane. So just the use of the negative birefringent compensation layer by itself is not sufficient to result in the state-of-the-art performances. In view of the above-mentioned LC structure symmetry issues the choice of the optimal technology for optical shutters for LCD light-switching elements is also very important, as the inherent birefringence and viewing angle changes with the LCD technology chosen. The fact is that the negative birefringent compensation layer takes care only of the effective birefringence of the central, homeotropically aligned part of the LC layer 3, while the effective birefringence of thin layers at the display boundaries 2a and 2b remain uncompensated (see FIGS. 1 and 2). Several technical solutions have been developed in order to compensate for this residual birefringence, however typically with the high definition, LCD panels (computer, TV, . . . ) in mind. They are based either on two and four-domain TN technology (IDRC 91, p 68, JJAP 34, p 2396 and number of different patents related to this technology), or on discotic LC polymer compensating film developed by Fuji Film (U.S. Pat. No. 5,559,618, U.S. Pat. No. 5,646,703, U.S. Pat. No. 5,525,265), already mentioned above, which allows for the use of the standard TN technology. The “2- and 4- domain TN solution” is not applicable for the LCD light-switching elements due to poor overall optical properties resulting from the alignment defects at the domain walls (high light scattering!) and therefore does not comply with international quality an safety regulations for personal optical protective devices like EN 379. The “Fuji solution” (U.S. Pat. No. 5,559,618, U.S. Pat. No. 5,646,703, U.S. Pat. No. 5,525,265), already mentioned above, though providing an elegant solution for the computer terminals, also does not seem to be very appropriate for the LCD light-switching elements. The reasons are threefold:                Inferior overall optical properties resulting from the light scattering in the discotic LC polymer compensation film (somewhat more than tolerated by the EN 379 international regulation),        Rather high price and very limited choice of the commercially available retardation values,        The residual birefringence in the boundary layers of LC changes with the driving voltage, as the effective thickness of these transitional regions 2a and 2b is voltage dependent, while the birefringent properties of the commercially available discotic films are fixed, being optimized only for the computer and TV display market.        
The optimum solution for the LCD technology used for LCD light-switching elements seems to be the one, in which the LC molecular alignment is as symmetric as possible and has self-compensating properties for oblique angle of light incidence. If these conditions are met, then the incorporation of a simple negative c-plate retardation layer between the LC cell and the crossed polarizers can result in wide viewing angle and high light attenuation. The introduction of “Pi-cell” by P. Bos from Tektronix (U.S. Pat. No. 5,187,603) represents an excellent example of such self-compensating LC alignment concept, which is schematically presented in the FIG. 3. FIG. 3a shows the molecular alignment of the homogeneously aligned LCD cell driven with the electric field with the amplitude noticeably higher than the switching threshold value (V˜3Vth). Comparing the effective birefringence for the light incoming at an oblique angle from the same direction as the LC molecular tilt on the alignment surface (incident light direction 7b) and from the opposite direction (incident light direction 7a), it becomes very evident that the light attenuation of such a LCD cell must be angular dependent, even if the central homeotropically aligned LC layer is compensated with a negative birefringent c-plate. On the other hand the same comparison made for the “Pi-cell” configuration, schematically shown in the FIG. 3b, clearly shows that the angular dependence of the front and back boundary LC layers mutually compensate each other. This basic principle has been in the past years upgraded by a number of additional technical improvements for various applications by Fergason (U.S. Pat. No. 5,515,186, U.S. Pat. No. 5,377,032) from OSD Envision and Welzen (EP 0638834) from Sagem, as well as in a number of different publications in Display Research scientific magazines (for example: P. Bos, et al: IDW '98, pp. 243-246 (1998) and JJAP 38, p 2837-2844 (1999), K. Vermeirsch et al SID 98 Digest, p 989, . . . ). The concept of the “Pi-cell”, angularly compensated with negative birefringent c-plate, probably represents the best-known technical solution for the wide viewing angle and high contrast LCD light-switching elements. It does however have some drawbacks:                Since the “Pi-cell” is operating on the principle of the electrically controlled birefringence (ECB), the production technical requirements are noticeably higher (cell homogeneity, cell thickness control, alignment requirements . . . ) than with standard TN LCDs—higher production costs,        In order to suppress 180° molecular twist formation, the “Pi-cell” has to be constantly electrically driven even in the “open” optical state and adjustments of the driving amplitude to obtain optimum transmission in the “open” optical state are necessary,        As the front and back boundary LC regions 2a, 2b of the LC layer (see FIG. 1) are aligned in the same direction, a reasonably large residual retardation is created. In order to achieve high light attenuation, extremely high driving voltages (40 V) are necessary unless an additional retardation layer is added, which compensates for the residual retardation. The need for the additional positive birefringent a-plate compensation layer, which is furthermore commercially not available, further complicates the construction and increases the production costs.        
Discussing various LCD technologies, adequate for the manufacturing the wide viewing angle and high contrast LCD light-switching elements, one has to mention also the Low Twist Nematic (LTN) LCD technology as described in the patents of Welsen (FR2728358) from Sagem, Nakagawa et al (U.S. Pat. No. 4,952,030) from Asahi Glass Co, Leenhouts et al (U.S. Pat. No. 4,609,255) from Philips, A. Hoernel et al (WO 97/15254, WO97/15255, PCT/SE95/00455) from Hoernel Intl. and published in a number of papers in the Display Research and Scientific magazines (S. Palmer, Appl. Optics, 36, No 10, p 2094, Hirakata et al SID 95 Digest p. 563, . . . ). Though this technology is used for mass-production of the welding light filters, one has to comment that the LTN technology in fact improves the viewing angle, since it allows for making thinner LCD cells and hence smaller effective positive-birefringence to be compensated, however the LCD cell construction is very asymmetrical due to low twist angles and the final results cannot be even close as good as with the “Pi-cell” solutions as described above.
Finally, evaluating various LCD technologies, one has to refer also to the supertwisted LCDs though they have never been used for light shutter applications so far (U.S. Pat. No. 4,634,229, U.S. Pat. No. 5,004,324, U.S. Pat. No. 5,155,608, J. Appl. Phys. 58, 3022, (1985), Appl. Phys. Lett. 50, 1468, (1987), . . . ). Being developed only with high multiplex driven, high resolution LCD display panels in mind (LCD laptop computers, TV, mobile phone displays . . . ) these displays are optimized for maximum steepness of the voltage response curve and so the LCD cell parameters (LC elastic constants, polarizer orientation, . . . ) cannot be the same as with the LCD light shutters, which operate with much higher driving voltages and require gradual voltage response characteristics in order to allow for voltage controlled light attenuation. Furthermore the laptop computer terminals require maximum brightness and tolerate an optimum viewing axis to be tilted to the display panel normal, while the LCD light shutters, which are typical on axis devices (glasses, helmets, optical elements, . . . ) do not. Finally STN LCDs are strictly based on the electrically controlled birefringence effect and are therefore typically two “eigen-mode” light-propagating devices, while it is advantageous for the LCD light shutters to use only single, preferably ordinary mode of light propagation (isotropic) as much as possible (for example TN LCDs) even at the expense of lower light efficiency in the open state in order to reduce the angular dependence of the light attenuation.